SAUDICAVES: How long have you
been working in the lava fields, Dr. Roobol?

DR. ROOBOL:We’ve been working in the lava fields of Saudi Arabia for the past 20
years. First we went to map them and date and analyze the lavas. Next we
returned for geohazard studies locating areas of microseismic activity and
geothermal phenomena. We went again to look for gemstones. After that, we
returned to site quarries for industrial materials such as for lightweight
aggregates and pozzolan and basalt for fusing to make rock wool and fused
basalt castings. Now with the Saudicaves project we have returned to locate
lava tubes and we are expecting to cooperate with the Department of
Antiquities to excavate caves with indications of occupation to levels of
ancient man whose stone tools abound on the desert surface. There are many
publications on the lava fields both in the international literature and in
internal reports. There are also a set of 1:250,000 colored geological maps
available through the Saudi Geological
Survey. [See references at the end of
this interview – Ed]

SAUDICAVES: Saudi volcanoes
are related to the opening of the Red Sea, aren’t they?

DR. ROOBOL:That is what most people think but it’s not that simple.The Red Sea is oriented NNW and there are two types of
basaltic lava-field in western Saudi Arabia.It is the older type which is deeply eroded and yields radiometric ages
of 15 to 30 million years, that are associated with the opening of the Red
Sea.These older lava-fields are
so deeply eroded that there are no morphological volcanoes left.

The volcanoes that we can see
today, e.g. by the side of the pilgrim expressway when we drive from Jeddah/Makkah
to Madinah, belong to a much younger set of basaltic lava-fields with age
dates from 10 million years right up to the historic eruptions.These young volcanoes and lava-fields form a distinct N-S chain in
western Arabia and the vents are aligned N-S, quite different from the Red Sea
direction.This chain is 600km
long and we have called it the Makkah-Madinah-Nafud volcanic line. It forms
the axis of uplift of western Saudi Arabia.Between Makkah and Madinah is the 20,000km2 Harrat Rahat lava-field
with 644 scoria cones, 36 shield volcanoes and 24 domes. Between Madinah andthe Great Nafud are the coalesced harrats Khaybar, Ithnayn and Kura
with an area of 20,560km2 and 327 scoria cones, 46 basaltic shield volcanoes,
20 domes, 5 tuff cones, one basaltic stratovolcano (Jebel Qidr which looks
like Mt. Fuji in Japan) and 39 massive and very long lava flows that we have
named “whaleback flows” from their unusual appearance in the field.These the biggest and longest basalt flows in western Saudi Arabia and
of course contain the biggest and most extensive lava-tubes.

All these volcanoes of the MMN
volcanic line are younger than 10 million years and they are NOT related to
the NNW opening of the Red Sea.They
do in fact represent a completely new N-S crustal rift that began forming only
10 million years ago in western Saudi Arabia.So in a few tens of millions of years there might be a new ocean along
this line similar to the Red Sea.

SAUDICAVES: What are the
characteristics of the lavas along the MMN Line?

DR. ROOBOL:All basalts look the same – black dense rock usually with a few shiny
crystals in long flows often about 10m thick with the famous pahoehoe
and aa surfaces (Hawaiian names for ropey or blocky
surfaces).However for the past
century petrologists have been looking carefully at the chemistry and
mineralogy of black basalt lavas and today the basalts of the different
tectonic settings such as ocean floors, continental rifts, hot spots and lunar
can be distinguished on their contents of such elements as Na, K, Y, Nb and Zr.The lava-fields of western Saudi Arabia are regarded as
forming in a continental rift setting and their chemistry is described as alkaline
in composition.However the lavas
and volcanoes of the MMN line are only mildly alkaline with modest Na and K
contents.The rock names for the
MMN line lavas are: alkali olivine basalt (AOB); hawaiite; mugearite;
benmoreite; trachyte and comendite.In contrast the young lava fields on either side of this line (such as
Harrat Kishb) are strongly alkaline with the rock names: alkali olivine
basalt, basanite, phonitephrite, tephriphonolite and phonolite.

Now, there’s a reason for
this. The lavas along the high axis represent a rift zone that is forming
today and within that rift, the magma is trapped inside the crust and
undergoes a process of differentiation (removal of olivine, pyroxene and
feldspar crystals) to produce extreme felsic compositions. In contrast, the
lavas on either side, such as Harrat Kishb, are very primitive. They are
mainly basanites coming direct from the mantle, without stopping on the way up
and transporting with them pieces of the mantle in the form of peridotite.

SAUDICAVES: Would these
differences in chemistry and mineralogy affect the creation of lava tubes?

DR. ROOBOL:No, this chemical difference is extremely subtle and we don’t expect
it to have any influence whatsoever on the formation of lava tubes and the
physical features of the flows and volcanoes themselves. It is the presence of
water and volatile components within the lava that controls viscosity and
physical forms. Water in particular can be picked up by lava near the surface.

SAUDICAVES: Where are we most
likely to find lava tubes?

DR. ROOBOL: The distribution
of the lava tubes is influenced by the physical features of the lava flows. We
have found in all the harrats along the MMN Line, that there are three
stratographic units. The oldest is characterized by extremely large volume
lava flows that cover great distances. They are of enormous volume and it is
in these large lava flows (which we have named “whaleback”) that you will
find the longest and the most frequent lava tubes. As the harrat evolves, the
lavas become smaller in volume with time and so the flows become shorter. The
volume erupted gets smaller and smaller. So maps of the harrats show three
stratigraphic units superimposed one on top of another. The youngest one has a
small area, the middle one has a medium area and the oldest parts of the
harrat extend furthest on either side.

So, it’s very much the older
parts of the harrats with these very large-volume and very long flows which
have the longest lava tubes within them. But there are many smaller lava tubes
in the youngest lava flows.

SAUDICAVES: How old are the
oldest ones?

DR. ROOBOL: It depends which
harrat you’re dealing with. In the case of Harrat Rahat, they would go back
to about ten million years. In the older parts of Khaybar to about five
million years and the older parts of Kishb to only two million years, based on
the existing potassium-argon dating.

SAUDICAVES: What is the age of
the lava tubes we found near Jebel Hil in Harrat Kishb?

DR. ROOBOL: The Kishb lava
tubes belong to the youngest stratographic unit which contains the shortest
lava flows. Jebel Hil and its lava tube is as young as 2000 years but the
adjacent eroded flows with caves may be as old as one million years. These
younger lava tubes are only partially filled with sand and debris. Where the
lava flows are five, eight and ten million years old, the lava tubes are
either eroded off the surface or filled with erosional debris.

SAUDICAVES: Isn’t it likely
that tubes in the older lava would have collapsed due to the lava breaking up?

DR. ROOBOL: I haven’t seen
that much erosion. No, I think the collapses on them are long chains of
collapse craters that probably date from the time of the eruption, when the
lava was removed from the tube and the roof collapsed from lack of support,
rather than late erosion. This is indicated by lava draping down into some ofthe collapses. The late erosion is still very slight.

SAUDICAVES: Can you tell us
something about Harrat Kishb where the first reconnaissance for lava tubes
took place?

DR. ROOBOL: Harrat Kishb is
one of the smaller lava fields in Saudi Arabia. It’s about 5,890 km2 in area
with 162 scoria cones 1 shield volcano, 9 domes, 6 tuft rings and 1 whaleback
lava flow. It has been divided into three stratigraphic units. It’s quite
difficult to date the oldest lavas because they’re covered in caliche and
salts caused by water evaporation. So we are not entirely delighted with our
numbers of samples dated, but the oldest rocks appear to be only two million
years old.

There’s a wide variety of
land forms on Kishb. They are mainly built up from monogenic – that’s one
eruption – volcanoes, producing scoria cones and basalt lava flows. But in
the youngest parts of the harrat, you get differentiated products of phonolite
domes with small areas of pyroclastic flows and fall fields around them.
It’s very distinctive, green weathering phonolite.

Harrat Kishb is very well
known in Arabia because of its abundance of peridiotite nodules carried in the
dominant rock type, which is basanite, a very primitive basalt. The
attraction, of course is that some of these peridiotite nodules carry large
crystals of gem-quality olivine and Saudi Arabia will shortly have a gem
industry, as we have drilled one prospect which has been licensed, which
contains 7,000 kgs of gem-quality peridot in a very small area, in gravels
less than seven feet thick.

SAUDICAVES: How about the lava
tubes on Harrat Kishb?

DR. ROOBOL:There are three lava-tube sites that have been identified on Kishb in
our preliminary reconnaissance. This is interesting, because they show that
the lava tubes formed in three different parts of a typical lava flow. The big
ones on Jebel Hil formed right next to the vent, and this is the simplest way
they form. Lava continues to flow after leaving the vent and the flow front is
fed by draining the near-vent area of the flow. So you’ve got a long, empty
tube forming next to the volcano.

The ones to the east of this
area formed in the middle of a flow, in a very interesting place, because the
lava flow divided around an old scoria cone, building up as a series of
arcuate ridges that wrapped around the older cone. These ridges are
topographically higher than the surrounding flow. Only the high parts of the
ridges drained to form local lava tubes, where lava has piled up behind an
obstruction.

The third cave, Dahl Faisal,
is formed somewhere in the middle of a very long flow. In this case, it was a
pahoehoe flow with a thin roof and as the flow was continuing to propagate,
the roof was inhaled as a conical funnel which popped at the bottom to form a
nice, smooth, round, funnel-shaped entrance and air was sucked into the
interior to form a lava tube only from that point downslope.

So,
there are three different mechanisms for the three localities that have been
found on the very first lava-tube reconnaissance of the lava fields.

SAUDICAVES:
Thank you very much for spending this time with us.

REFERENCES

The
scientific results of the petrology of harrats were published in the
international scientific literature available in geology and earth science
libraries around the world.The
colored maps are on sale at the Saudi Geological
Survey.Data on industrial uses, gemstones, seismic hazard, geothermal
phenomena and lava tubes remain in internal reports of the Deputy Ministry of
Mineral Resources, Jeddah, some of which are confidential.A full set of these internal reports are available in the library of
the Saudi Geological Survey, Jeddah.